The present invention relates to a method and apparatus for active control of acoustic output in gradient coils and more particularly for reduction of the noise output of a coil system without significant interference with the magnetic field generated by the coil system.
As magnetic fields get higher in magnetic resonance imaging, (MRI) (P. Mansfield P. and P. G. Morris, NMR Imaging in Biomedicine. Academic Press, NY. (1982)), the gradient coils necessary in the imaging process become more obtrusive by virtue of the very high levels of acoustic noise generated. This arises because the very high currents in the wires required to produce the gradients generate very large Lorentz forces by interacting with the large static magnetic field present.
Various attempts have been made to ameliorate the acoustic noise is problem by damping, or by attempting to cancel the noise intensity in air. This approach is like treating the symptoms of the problem rather than going to the cause. More recently a new approach has been advocated which balances all Lorentz forces within the coil system (P. Mansfield, B. Chapman, P. Glover and R. Bowtell, International Patent Application, No. PCT/GB94/01187; Priority Data 9311321.5, Jun. 2, (1993), P. Mansfield, P. Glover and R. Bowtell, Active acoustic screening: design principles for quiet gradient coils in MRI. Meas. Sci. Technol. 5, 1021-1025 (1994) and P. Mansfield, B. L. W. Chapman, R. Bowtell, P. Glover, R. Coxon and P. R. Harvey, Active acoustic screening: Reduction of noise in gradient coils by Lorentz force balancing. Magn. Reson. Med. 33, 276-281 (1995)). In principle this should solve the problem completely. This would be the case if the supporting material coupling the force balancing wires had an infinitely high compressional wave velocity. Unfortunately, from this point of view, all materials are compressible and for most readily available plastic materials the sound wave velocity lies in the range 1-3 kmsxe2x88x921. In practice this means that when frequencies increase and, of course, depending on the dimensions of the supporting material, the structure can approach a resonance condition. But even before this occurs it has been shown quite recently that distortions in the supporting material are responsible for the launch of sound waves in air (P. Mansfield, P. M. Glover and J. Beaumont, Sound Generation in Gradient Coil Structures for MRI. Magn. Reson. Med. (to be submitted)). For a flat plate it is shown that the sound waves in air produce an acoustic diffraction pattern. When Lorentz force balanced wires are coupled with any material the sound output depends on frequency and is generally large unless additional precautions are taken to absorb acoustic energy in the diffraction side lobes. This approach is, therefore, likely to be of limited value only. In a recent PCT application No. GB96/00734, a different approach has been advocated in which additional wire loops are added as an integral part of the coil structure in an attempt to cancel the acoustic wave generation within the solid structure. This approach exploits the Lorentz forces which are generated by the additional loop of wire. When the phase and amplitude of the Lorentz forces are adjusted correctly this approach appears to produce the desired acoustic noise cancellation. A slight disadvantage with this approach is that the additional wire loops will themselves produce undesirable magnetic fields at the gradient coil centre. For magnetically screened coils the additional wire loops can also produce undesirable extraneous magnetic fields which to some extent vitiate the magnetic screen.
In the new approach described in this invention the additional wire loops mentioned above are replaced by self-contained transducers which may be of the piezoelectric type or some other type and which produce no substantial extraneous magnetic fields.
Quiet gradient coils will be of value in a range of MRI applications where ear-plugs or ear-defenders for the subject are either undesirable or impractical. Such situations exist in foetal scanning in pregnancy, paediatric imaging of the very young and in veterinary applications. Wider applications exist with normal patient scanning where noise levels, even with ear-plugs, become intrusive and potentially dangerous. Quiet gradients could find a special application in studies of functional imaging of the brain. There is considerable interest in studying the acoustic cortices and their response to specially tailored phonic inputs. Such responses can be overwhelmed by the intrusive gradient noise input unless it can be significantly attenuated.
U.S. Pat. No. 5,548,653 discloses an active noise and vibration actuators which are mounted on support members on the surface of a coil structure and are activated to minimise the total noise eminating from the system.
The present invention provides apparatus for active acoustic screening of a coil system, said coil system comprising at least one current carrying conductor, the coil system being suitably mounted either between two blocks of acoustic material, the blocks being separated by a gap or within a slot in one block of acoustic material with a slot within the block, characterised in that an active electromechanical transducer is mounted in said gap or slot in order to balance the active and reactive forces produced by the transducer and including means for energising the electromechanical transducer to oppose the vibratory noise generated by said coil system when energised in a static magnetic field.
The present invention also provides an active acoustically controlled magnetic coil system comprising at least one current carrying conductor configured in a loop to provide outward and return paths characterised in that said outward and return paths are mounted in support blocks of material having predetermined acoustic transmission characteristics, said support blocks comprising first and second portions separated by a gap said outward and return paths being mounted respectively in said first and second portions and further including at least one active transducer mounted in said gap, and comprising means for supplying a predetermined alternating current to said conductor and energisation means for supplying the at least one active transducer with selected energisation and selected phase with respect to the said alternating current, said energisation being selected to generate mechanical movement in said transducer to actively oppose vibrations created in said support block by said conductor to reduce the acoustic noise generated by the coil system.
The present invention further provides an acoustically controlled magnetic coil system in which there are provided a plurality of transducers, each being provided with energisation to create mechanical movement within the transducer to reduce the noise generated by the coil system when energised in a static magnetic field.
Preferably the means for determining the selected energisation supplied to the transducers comprises a microphone mounted to receive noise signals generated by the at least one current carrying conductor, said microphone being connected to a display means and including a control circuit supplying relative phase and amplitude control signals to said energisation means to energise the transducers to reduce the noise generated by the coil system.
Preferably the active transducer comprises a piezoelectric transducer. The piezoelectric material may be a quartz crystal, barium titanate, lead zirconium titanate or other such piezoelectric materials.
Alternatively, the active transducers may comprise magneto strictive devices of a size small enough so as not to significantly affect the required gradient field. A further alternative is an active transducer comprising a thin current loop enclosing a rubber block, its dimensions chosen so as to make the device non-resonant at the operating frequency.
In a first embodiment, the first and second portions of the support block may comprise the same material and the position of the gap may be equidistant between the outward and return paths.
In a second embodiment the first and second portions of the support block may comprise different materials having different acoustic compressional wave velocities and different attenuation constants, the position of the gap may then be selected to be non-equidistant between the outward and return paths of said current carrying conductor.
In a third embodiment, the transducer may be provided with a matching unit to enable it to produce different mechanical movements on each side of the gap.
In a further embodiment the outward and return paths may comprise separate conductors with separate drive means forming a coil system comprising a primary coil and a magnetic screening coil for said coil system, the transducer means being energised to reduce the acoustic output of the coil system.
In a complex system for MRI the coil system may comprise x,y and z gradient coils and screening coils for said x,y and z gradient coils and the transducers may be mounted between said coils and said screening coils and energised to reduce the acoustic output of the system.
The transducers may require to be spatially distributed with common zones used to cancel the acoustic wave for the x,y and z gradient coils and a logic matrix drive system may be used to selectively energise a plurality of drive units for the transducers, those transducers within any common zone area being driven at an amplitude and phase which may be different to those in non-common zones.
Preferably the conductors for each primary coil and magnetic screening coil and the acoustic control transducers are each distributed on individual cylindrical geometries, each of the aforesaid coils being arranged in the form of a fingerprint coil each cylinder being arranged in a nested concentric and coaxial set.
When wires are pulsed with current, I, within a magnetic field, they experience a Lorentz force which depends on the magnetic field strength, B, and is maximised when the angle between the current and field directions is 90xc2x0. If the wire is embedded in a supporting block of material, its movement will launch an acoustic wave in the material. The basic idea for this invention is to cancel the acoustic waves which are launched by the gradient coil wires into the supporting structure. But this is done by injecting a suitably phased second acoustic wave of appropriate amplitude to cancel the first wave within the solid structure. In a previous design this is achieved by adding extra wires to the gradient coil design (P. Mansfield, Active Acoustic Control in Quiet Gradient Coil Design for MRI. PCT Application GB96/00734). In the present invention this is achieved by use of suitable non-magnetic transducers. These transducers could be piezoelectric transducers based on either quartz crystals, barium titanate or lead zirconium titanate or other such piezoelectric materials, or by the use of suitable magneto strictive devices which are small enough so as not to affect significantly the required gradient field or by small self-contained, localised current loops arranged so that the loop plane is orthogonal to the magnetic field B. In the following we shall use the word transducer to mean any one of the above mentioned devices or a device working on similar principles singly or in combination with any of the aforementioned devices.